Though wind energy is increasingly popular, especially with the threat of global climate change, the cost of energy from wind farms is still not competitive with that of more conventional power sources. Additionally, most of the top-tier wind farm sites have already been taken, forcing new developments to move to less favorable environments which will make the large scale deployment of wind energy all but impossible with current technology.
Windmills in recent years have become more effective and competitive with other energy sources, but most still remain very expensive to install. As a result, their overall cost per installed kilowatt (kW) is still high enough that they are only marginally deployed and contribute only a small amount to the “electrical grid.”
The primary configuration of modern windmills is a horizontally-mounted, large diameter, three-bladed propeller that rotates at low revolutions-per-minute (rpm's) over a very large swept area. The higher the rotational axis of the propeller can be mounted, the better, as the natural speed of the wind increases with an increase in the height above the ground. Conventional windmills thus have very tall and very strong tower structures. Typically, they have a tubular steel tower that is mounted to a deep, subterranean cement base. The system has to be very carefully engineered and sited appropriately for the surrounding terrain. The towers must maintain a central stairway or other means to allow construction and operator access to the upper mechanicals. The tower must accommodate the heavy gearbox, electrical turbine, and propeller assembly, as well as be strong enough to withstand gale force winds, and potentially earthquakes. To make the system even more complicated, the upper nacelle and gearbox/turbine housing must be able to pivot on a vertical axis, so as to align the propeller correctly with the wind direction at any time during the day or night.
On many windmill systems the individual blades of the windmill are able to rotate about their individual longitudinal axis, for pitch control. They can optimize the pitch of the blades depending on the nominal wind speed conditions that are present at anyone time at the site. They can also change the pitch of the blade to “feather” the propeller if the nominal wind speeds are too large. Occasionally the windmill is locked to prevent rotation, and the blades feathered to prevent major damage to the machine in a storm. All of this pitch control technology adds significantly to the cost of windmills.
Another major disadvantage with conventional windmills is damage caused by lightning during thunderstorms. The blades can be upwards of 300 feet in the air and are a good source for lightning to find a conductive path to the ground. Some of the more recently designed windmills use a system of replaceable sacrificial lightning conduction attractors that are built into each windmill propeller blade. They help channel the lightning away from the vulnerable composite structure that comprises the blade itself. The fact remains that one of the major causes of windmill downtime and maintenance costs are caused by lightning damage.
The size of many windmills is also a major problem for inspection, diagnostics, and repair. Often workmen have to use ropes and climbing techniques to perform maintenance on the massive machines. It is very expensive and dangerous. In recent years workmen have fallen to their death trying to repair the blades.
There have been a number of proposals for more efficient and/or cost effective means of harvesting the wind's energy in order to combat the high price of wind energy. There has been considerable effort put into developing diffuser-augmented wind turbines, which have considerably higher power output for a given size rotor than conventional turbines. However, the cost of the diffuser has not justified their commercial implementation.
Some effort has also been made to develop high-altitude wind harvesters, as high-altitude winds are considerably stronger than ground level winds and are present almost everywhere. In one example of this effort, it has been proposed to provide tethered wind turbines that are deployed at or above ground level. See U.S. Published Application 20080048453 to Amick, the disclosure of which is incorporated herein by reference in its entirety.
However, no conventional windmill yet addressees the foregoing problems while providing for cost-effective wind-energy production.